1. Field of the Invention
This invention relates to data processing apparatus and methods, and to apparatus and methods for detecting the content of impurity in a gaseous medium.
2. Prior Art
The problem of detecting a fire by means of smoke detection is not entirely solved by using a very sensitive aspirating detector.
Where high-valued property is being protected against fire, early detection of the fire is essential; allowing time for it to be tackled with the minimum of damage being caused. The build-up of a fire may be very gradual or very rapid. Electric power cable or electronic components which become overloaded result in charring or smouldering of surrounding material long before any `red-hot` fire occurs. The cause of many fires is localised over-heating which slowly builds to a full-scale conflagration. In these cases the product of overheating is small and a high-sensitivity transducer is required to respond to it. The build-up of fire may also be seemingly instantaneous, as with an explosion. No matter which way a fire starts, early detection is essential for it to be effective and fast generation of an alarm is a normal requirement. There are two main requirements for early detection. The first is a high-sensitivity transducer by which very small amounts of the product of overheating can be detected. The second is rapid recognition that the output signal from the transducer is in fact due to the product of overheating and not a signal which could be generated from any other source. Basically, a detector consists of a transducer giving a signal proportional to the product of overheating and a device for giving an output signal when the transducer output signal has reached a given alarm level. The general environment for a detector is one where a considerable variation of signal from a high-sensitivity transducer will be normal. High sensitivity alone will either produce a fire signal due to this normal background activity or the given alarm level must be raised to a point where the normal background activity will be ignored, thus de-sensitising the detector. This places a practical limit on the sensitivity of the detector.
In order to prevent the internal parts of a transducer from becoming contaminated, an air filter is employed to remove larger particles than anticipated with products of overheating materials. The performance and renewal of this filter, with known aspirating detectors, is often a problem in maintaining a consistent performance by a detector. The problem arises in that, as the air filter becomes increasingly contaminated through usage, the effective pore size of filter medium becomes increasingly smaller. This effect is sometimes known as "tea leafing" because of the similarity with the effect of tea leaves hindering passage of tea when a tea strainer is used. The effect on conventional aspirating smoke detectors is that the sensitivity of the detector decays with time as the filter becomes increasingly blocked, leaving the system in some cases significantly less sensitive after only a few months of use.
Furthermore, in existing aspirating smoke detection systems, it is desirable that the rate of air flow down the sampling pipes is at a maximum, because this minimises response time to fire situations. By reason of the high rate of flow down the sampling pipes, it is also likely that significant amounts of pollution other than smoke and having the aforementioned tendency to block filters will be drawn into the detector. "Referencing" is the process by which an air pollution transducer of an air pollution detection unit samples the air entering a protected volume and provides an offset signal to other air pollution detection units within the volume. The intention of referencing is to avoid the problem of pollution entering the volume from outside causing an alarm. There are two possible roles for the air pollution detection unit in referencing. The first is to provide a reference signal and the unit is called a reference unit; the second is to provide acceptance for a reference signal and the unit is called a detector unit. "Referencing" is generally applied in circumstances where the unpurified external atmosphere has access to the protected volume, for example via ducting or a window. It is not needed in circumstances where the external atmosphere does not have access to the volume or is purified before it reaches the volume.